The performance of magnetic heads is dominated by the accuracy with which the gap can be formed. With a small size or miniature magnetic head to which the invention is directed, the gap length is as small as of the order of 1 to 2 microns, and it is extremely difficult to form a gap of such length with a high degree of accuracy. In the prior art practice, a gap has been formed by disposing a pair of core block pieces in abutting relationship so that their gap defining surfaces are located opposite to each other, with a spacer comprising a thin sheet of mica or metal foil interposed therebetween. A gap forming material is caused to melt and permeate into a clearance formed between the two gap defining surfaces by the presence of the spacer, and subsequently solidified. With this technique, there results a high manufacturing cost, inasmuch as the spacer material itself is expensive and a considerable difficulty is experienced in machining the spacer to a thickness of 1 to 2 microns with a high accuracy.